The present invention concerns an arrangement for underwater drilling of foundations and, more particularly, an arrangement of the type comprising at least one cutter and a pump for discharge of the excavated material, and hydraulic motors for driving the said cutter and the said pump.
The exploitation of hydrocarbon beds at great depth increasingly calls for production platforms requiring the use of very heavy duty anchoring points. This is the case, for example, in so-called "taut cable" platforms.
Such anchoring points, capable, for example, of absorbing vertical or horizontal forces of the order of 1000 or 2000 tons, require construction in the bottom, under several hundreds of meters of water, of piles several square meters in section and several dozen meters in height, and therefore require the construction of corresponding boreholes.
Since rotary drilling is excluded for constructing such anchoring points, other solutions have already been proposed.
Thus, French Patent Application No. 85 14939 describes an arrangement comprising:
a drilling apparatus having a head linked by a hydraulic advancing jack to a member equipped with cutters, said apparatus being additionally equipped with a pump for discharge of excavated material and with hydraulic motors for driving the cutters and the pump;
a rigid tubular assembly comprising a pipeline for discharge of excavated material and pipelines for supplying the jack and the motors with hydraulic fluid;
means of connection between the head of the drilling apparatus and the lower end of the tubular assembly;
means of support placed on the sea floor in drilling head, designed to support means of locking the tubular assembly and
means of suspension of the arrangement from a ship; means being provided for the supply of hydraulic fluid, and connecting pipes being provided between these means of supply of fluid and the end of the tubular assembly.
This arrangement, however, is limited, at very great depth, by the weight and the cost of the tubular assembly, and by the power required to secure the circulation of hydraulic fluid.
The present invention is designed to furnish a drilling arrangement better suited to very great depths.
For this purpose, the subject of the invention is an arrangement for underwater drilling of foundations, comprising at least one cutter and one pump for discharge of excavated material, and hydraulic motors for driving the said cutter and the said pump, characterized in that it comprises an hydraulic turbine capable of functioning by means of a fluid such as sea water, and an hydraulic power plant driven by the said turbine and designed to feed the said hydraulic motors.
Unlike heretofore, the arrangement according to the invention therefore comprises its own power plant, consisting of the hydraulic plant. The primary energy is supplied very simply from the service ship in the form, for example, of sea water under pressure, which may be obtained, for example, by means of mud drilling pumps with which all drilling ships are equipped.
In addition, the power necessary is considerably reduced since there is no longer any circulation of hydraulic fluid between the arrangement and the surface, but only injection at the inlet of the turbine of fluid such as sea water under pressure, which is then evacuated at the bottom of the borehole.
Moreover, it will be noted that the arrangement according to the invention requires no transfer of electric power between the surface and the bottom and consequently avoids the use of a heavy duty cable which presents problems difficult to solve because of the depth of exploitation considered.
In a preferred embodiment, the arrangement according to the invention comprises two cutters with horizontal shafts turning in reverse direction.
To accomplish regulation of the speed of the turbine as simply as possible, there may be provided a discharge valve opening under the effect of an increase in pressure at the inlet to the turbine caused by excessive speed.
The discharge of the turbine preferably is directed over the tools of the cutter, so as to secure cleaning thereof.
The hydraulic plant may comprise a plurality of hydraulic pumps, for example one per motor, driven by the said turbine by way of a mechanical distribution box.
In one particular embodiment, the arrangement according to the invention may comprise a head forming a first distribution box for feed of the said turbine and discharge of the excavated material.
This head may be linked to the body of the arrangement by at least one advancing jack fed by the said hydraulic plant.
The head may in addition be linked by a two-line pipe to a second distribution box connected, on the one hand, to means for supplying the fluid feeding the turbine and, on the other, to a hose for evacuating excavated material.